1. Field of the Invention
The present invention relates to a dimmable control circuit, and more particularly to a dimmable control circuit having a first resistor and a second resistor, both with the negative resistance temperature coefficient, to achieve low temperature startup and high temperature operation.
2. Description of the Related Art
At present, the common dimmable electronic ballast usually works on the brightness control of the fluorescent tube in an order from dark to bright. That is, when the user turns on the power switch having a coactive variable resistor, the ballast will first work in low resistance of the variable resistor causing dim brightness of the tube, and then work in higher resistance of the variable resistor causing the tube brighter as the user turns the variable resistor clockwise. However, there exist flaws when the ballast activates the tube in low resistance of the variable resistor.
Besides, the dimmable control chip of the common electronic ballast, taking the PHILIPS's chip UBA2014 as an example, but not limited thereto, usually connects its reference voltage terminal with a set of normal resistors with positive resistance temperature coefficient to produce a corresponding reference voltage (VREF). When the switch is just turned on, due to the extremely low resistance of the variable resistor, the input voltage to the VREF terminal will be lowered down, causing the operating frequency of the UBA2014 to rise up to about 80 KHz and providing the tube with the minimum power. When the resistance of the variable resistor is turned to its maximum around 10 KΩ, the input voltage to the VREF terminal will go high, causing the operating frequency of the UBA2014 to go down to about 40 KHz and providing the tube with the maximum power.
However, when the surrounding temperature goes down to, for example 0° C., the resistance of the known voltage dividing resistors having positive resistance temperature coefficient goes down as well, causing the reference voltage too low to activate the UBA2014. Further more, when the surrounding temperature goes up, the resistance of the known voltage dividing resistors having positive resistance temperature coefficient goes up as well, causing the reference voltage to rise up and as a result the output operating frequency of the UBA2014 will be lowered down, and the power consumption of the tube will increase. These are cons of the known dimmable control circuits.